Possible idea for 4.9 brainstorming (but more likely for 4.10 or later): I’ve got a number of ideas for a reworking of export/import, spurred in part of some work I’ve done on the WordPress-Importer redux plugin over on GitHub.

For starters:

make the importer fully XML namespace-aware;

using `XMLWriter` to generate the export markup (instead of `echo` statements (this would probably also apply to all the RSS/Atom feed code as well);

hooks in the exporter (and corresponding hooks in the importer) to make it possible for plugins to add export/import additional information (e.g., info from private tables that is associated with posts/terms/authors).

a reworking of the WXR XML markup (to make both namespace-aware processing and a true streaming importer easier);

publish documentation on WXR, preferably in the form of an XML Schema. I’ve produced a sample schema that can be found at WXR. [Note: as mentioned in the readme there, the schema in NOT intended for run-time validation, but only to document the markup of WXR.]

One small thing: sometimes when you get an update notice for something, you realize you don’t need it. (Like old twenty* themes.) Ability to delete a plugin or theme from the updates screen, instead of updating it.